There is a need to measure corrosion on aircraft, bridges, buildings and manufacturing and process control equipment. As an example of the magnitude of this problem the U.S. Air Force estimated in 1991 that the cost of treating corrosion on its aircraft was approximately $718,000,000. The costs associated with commercial aircraft, bridges and buildings in the U.S. alone are expected to be significantly larger. To address this problem in a cost effective manner, means are needed to nondestructively test for corrosion at many key points over distances that in some cases can be hundreds of meters.
For many structural applications, the use of fiber gratings has been proposed as a method to measure strain. A great deal of work has been done investigating the usage of fiber gratings to measure axial strain and temperature. In many cases where the fiber gratings are embedded into structures, the fiber gratings are exposed to transverse strain that can result in a spectral shifts of magnitudes approaching the spectral shifts caused by longitudinal strains and/or temperature shifts. E. Udd, in U.S. Pat. No. 5,591,965, which issued Jan. 7, 1997, describes a three axis strain and temperature fiber grating sensor formed by writing two overlaid fiber gratings onto a birefringent fiber. As an example wavelengths of the fiber gratings can be written at 1.300 and 1.550 microns. By writing onto the highly birefringent fiber four gratings are established. In the case of about a two millimeter beat length at 630 nm they would be at 1300.0, 1300.6, 1550.0 and 1550.8 nm. The birefringent axes are well defined so that transverse strain can be measured along with longitudinal strain and temperature through four equations with four unknowns.
Further improvements in the measurement of transverse strain were made by E. Udd in patent application U.S. Ser. No. 08/707,861 entitled, "Transverse Strain Measurements Using Fiber Optic Grating Based Sensors", filed Sep. 9, 1996 now U.S. Pat. No. 5,828,059. These patent applications describe transverse fiber grating strain sensors that have improved transverse strain sensitivity and greatly reduced thermal sensitivity.
Fiber optic corrosion sensors that have been proposed often rely on spectral analysis of chemical analytes placed at the end or in the evanescent field of an optical fiber. When corrosion is present the spectral signal changes. The corrosion sensors based on chemically induced changes in the spectrum are difficult to multiplex, expensive to implement and are subject to erroneous signals due to the presence of chemical contaminants that do not represent corrosion, but induce spurious signals.
What is needed is a corrosion sensor that is compatible with a fiber grating strain sensor system usable for structural health monitoring, which also serves to provide necessary state of corrosion information.